1. Field of the Invention
The present invention relates to wavelength division multiplexed optical communication systems in general and, more particularly, to wavelength division multiplexed optical communication systems having individual optical channels which are capable of simultaneously supporting multiple electronic data formats such as TDM, ATM, and IP.
2. Description of the Related Art
As the need for communication signal bandwidth increases, wavelength division multiplexing (WDM) has progressively gained popularity for multiplying the transmission capacity of a single optical fiber. A review of optical networks, including WDM networks, can be found in Ramaswami et al., Optical Networks: A Practical Perspective (Morgan Kaufmnan, .COPYRGT. 1998), the disclosure of which is incorporated herein by reference. Typically, wavelength division multiplexed optical communication systems have been designed and deployed in the long-haul, interexchange carrier realm. In these long-haul optical systems, a wavelength division multiplexed optical communication signal comprising plural optical channels at different wavelengths travels in a single direction on a single fiber (unidirectional transmission). Because the communication traffic in such systems commonly travels many hundreds of kilometers, the need for add-drop multiplexing of individual channels is infrequent, occurring at widely-spaced add-drop nodes.
Although the optical infrastructure of long-haul WDM optical systems can accommodate future traffic needs created by increased demand from traditional and multimedia Internet services, this traffic must first be collected and distributed by local networks. Currently, such local networks are structured to carry a single wavelength, time-division multiplexed (TDM) optical signal along a fiber network organized into various ring structures. To route the various components of the TDM signal, numerous electronic add-drop multiplexers are positioned along the fiber network. At each add-drop location, the entire optical signal is converted into an electrical signal; the portions of the electrical signal which are destined for that add-drop point are routed accordingly. The remaining portions of the electrical signal are converted back to a new TDM optical signal and are output through the electronic add-drop multiplexer. Thus, before a user can access the bandwidth-rich WDM long-haul transport networks, he must first pass through the bottleneck of the local networks.
To increase capacity on these local networks, e.g., by using higher-rate optical transmitters, all of the equipment positioned on an optical ring must be upgraded. Further, providing additional add-drop nodes along a ring requires a re-examination of the optical power budget for the entire ring structure. Although WDM may be "overlaid" on such a local network to increase capacity, an all-optical solution is insufficient to meet the needs of future service demands. In particular, conventional WDM networks cannot handle the rigorous add-drop requirements of local networks to provide adequate routing of traffic. Further, current WDM solutions do not address the problems posed by the need to carry traffic having various data formats such as TDM, ATM, IP, MPLS, etc. simultaneously on the same optical network.
Several attempts have been made to remedy the problems of conventional optical networks. In U.S. Pat. No. 5,751,454, a wavelength bypassed ring network is proposed in which the wavelength channels are arranged so that some bypass each node and terminate further along the ring. Signals on bypass routes are not processed by intermediate nodes. While this system allows for fixed WDM add-drop on ring networks, it does not address the need for various data formats to be able to access the optical network.
U.S. Pat. No. 6,069,892 describes a wavelength division multiplexed optical communication system configured to carry fixed-length cells such that the system is optimized as an ATM cell transmission system. Because this system is optimized for ATM traffic, each optical channel of the WDM signal carries cell-based data, i.e., data having a single format. While such a technique enhances the use of wavelength division multiplexing with cell-formatted protocols, the formats for other protocols are not carried by the system.
In U.S. Pat. No. 6,084,694, a WDM communications network having a plurality of nodes is described. The wavelengths carried by the network are organized into wavebands of four channels; each node includes a filter for statically dropping a waveband and passively forwarding the remaining bands. To create what is termed a "protocol independent" network, each optical wavelength may be connected to a different data source. Thus, as shown in FIG. 9 of the patent, a SONET OC-3 signal may be sent from node Z to node B without conversion to an electrical signal by intermediate nodes. While the '694 patent depicts potential solutions to some optical network problems, it does not describe a system with sufficient flexibility to route any type of data format onto any channel wavelength and deliver it to any node within the optical network.
Thus, there is a need in the art for a wavelength division multiplexed optical network which is capable of transporting multiple data formats simultaneously on an individual optical channel. Such an optical network would impart the flexibility required to provide access to any type of data format to any customer at any point along an optical network.